Has to be Hammond or Lesley. Yes, use 2K5. A 50W part will need BIG heatsink. And if it burns-out, the 6V6es will be under-biased and red-plate fast. Depending how long it must run, I'd be looking at plain dumb ceramic stick rated 100W. You could instead omit the FC and raise the 75 ohm to 125-150 ohms. The lower supply current will un-sag the B+, so you better compute plate dissipation. And be wary of very-old 6V6 over 330V. No capacitor on the screen node?
1. With four new Electro-Harmonix 6V6GT tubes and changing the output cathode resistor to 150 ohms, what, if anything do I need to do on the plate side? The output transformer is 4KCT impedance with a DC resistance of 75 ohms per half. (PRR: you were referring to a straight cathode resistor to ground instead of the fixed bias, right?)
2. What should I hang on the screen grids? Assuming a voltage divider, but what kind of current?
3. What effects would unsagging the B+ make on the preamp* section and detract from any tone-desirable distortion characteristics?
What changes would be advised to mitigate that?